The long-term objective of the proposed research is to define and characterize the molecular mechanisms underlying genetically-determined resistance and susceptibility in snails Biomphalaria glabrata to infection by larval Schistosoma mansoni, causative agent of human schistosomiasis mansoni. Defined susceptible (S) and resistant (R) strains of B. glabrata will be employed in comparative biochemical and immunological studies designed to provide an indepth analysis of the molecular interaction between snail hemocytes (primary anti-parasite effector cells) and the early intramolluscan stages of S. mansoni (miracidium, primary sporocyst). Surface polypeptides from S and R strain hemocytes will be labeled using a highly sensitive biotin-avidin system and subjected to SDS-PAGE analyses to compare the occurrence of individual surface polypeptides on cells of each host strain, and to determine which of the identified components serve as binding receptors for larval excretory-secretory proteins (ESP) or sporocyst tegumental surface proteins (TSP). S and R snail hemocyte cDNA expression libraries also will be constructed, and screened with hemocyte membrane-reactive antisera to identify cDNA inserts encoding potential cell surface polypeptides. Expressed recombinant hemocyte proteins will then be used in further structural comparisons between snail strains, and tested for their abilities to bind S. mansoni ESP/TSP and to alter hemocyte function as a consequence of binding. Finally, individual ESP and TSP components will be isolated by high performance liquid chromatography and preparative PAGE, and evaluated for their ability to bind S and R hemocyte surface proteins (receptors) and to modulate hemocyte-sporocyst interactions. this focused research plan directly addresses questions related to the biochemical basis for hemocyte recognition and/or activation by early schistosome larvae. It is anticipated that the information generated by this research will provide important insights into the basic mechanisms of pathogen recognition by the molluscan internal defense system, and eventually may be applied to novel approaches to control invertebrate vectors of human disease.